拜託一下大家了，想好久想不通。
Show that for any closure operation on(P(A), C_) which satisfies 44, one has
phi bar=phi
Hint: Consider the empty family of subsets
解釋一下
bar:是指那個operation，在紙上寫就是上面加一槓。
C_:是集合裡被包含的符號
44;for any finite family E C_ P(A), one has (supE)bar=sup(E bar)
希望大家看的懂符號= =''
還有一題
A partially ordered set (S,<=) is called a pre-lattice if every nonempty
finite subset E C_ S has supremum and infimum. Show that (S,<=) is a
pre-lattice if and only if, for any s,t 屬於S both sup{s,t} and inf{s,t}
exist.
多謝大家啦
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